In a sequential chrominance and memory device television system, viz SECAM, the subcarrier is alternately frequency modulated by weighted chrominance signals so that the transmitted subcarrier alternates between such weighted chrominance signals; i.e., color difference signals, at the line period of 64 microseconds (1/15625Hz). The utilization of such a transmitted signal requires that the utilization device separate the luminance and subcarrier signals, then demodulate the subcarrier and matrix these signals. Because the transmitted subcarrier alternates at the line rate, the necessary second chrominance signal is received from a memory device which is storing the subcarrier received on a preceding line. A color recognition signal is radiated during each field blanking period and this color identification signal is utilized by the utilization device to synchronize a line rate switch which feeds the delayed and direct subcarrier signals to appropriate demodulators hence correct inputs to the matrix. (See "Color Television", Vol. 2, "PAL, SECAM AND OTHER SYSTEMS", first published in 1969 by Iliffe Books Ltd. for WIRELESS WORLD, copyrighted by P. S. Carnt and G. B. Townsend, 1969, pages 179-220.)
The chrominance subcarrier of the aforementioned television system can vary over a wide range of amplitude which requires, at the receiving end, a high gain limiter to maintain the output of the discriminators within the limitation of the system, which is defined as the ratio of the nominal operating voltage at the input of the decoder to the voltage corresponding to a specified percent reduction of the demodulated signal amplitude. As the noise threshold of a high gain limiter is quite low, when no signal is present at its input it is extremely difficult to determine if the system is exhibiting a poor signal-to-noise ratio or no signal at all. This, of course, produces immediate and objectionable effects which are presented the viewer of this system if a mistake is made in the determination as to whether the system is exhibiting no color signal at all or exhibiting a poor signal-to-noise ratio. Vertical line identification signals were added to SECAM to overcome this disadvantage. Proposed changes in the composite SECAM signal format (deletion of these vertical line identification signals) are reemphasizing this problem, but there has not yet been an effective solution presented.
Additionally, as previously discussed, most SECAM decoders, and encoders, utilize the color identification signal present during the vertical interval, such signals currently being phased out of the composite SECAM system thereby leaving only a white reference present on the back porch for line sequence determination and color enable. Utilizing the white reference for color synchronization, however, results in the same noise immunity problem as in detecting chrominance. Because of errors which are common to the insertion of vertical line identification signals, color synchronization based on the color subcarrier during the back porch interval may give different results than using the vertical line identification signals. Therefore, synchronization circuits of the utilization device should provide system operator flexibility for detecting color sequence by any of the above discussed methods, such circuits heretofore ineffective because the composite SECAM signal will not always meet the tolerances required.
As is also well known, numerous principles applicable to most, if not all, television systems in use today such as NTSC, PAL, SECAM, ART, NIR, etc. have been laid down. For example, in the SECAM system the chrominance subcarrier is suppressed during an interval of time, namely 5.7.+-.0.3 microseconds, such interval of time beginning with the line suppression signal and terminating after the lead-in of the synchronization. (See "SECAM Colour T.V. SYSTEM", Imprimerie NORD-GRAPHIQUE, Paris-10.degree..) Such principles require complex circuits for timing, etc., thereby increasing the cost and complexity of the system.